This invention relates generally to coating systems for metallic substrates, and more specifically to nickel aluminide coating systems and coated articles.
Nickel aluminide (NiAl) type coatings are discussed in U.S. Pat. Nos. 6,620,524 and 6,933,058 and US Patent Publication, 20070207339. Exemplary processes to provide a NiAl based overlay coating include cathodic arc (ion plasma) deposition techniques using sources having a generally homogenous composition. This process results in uniform chemistry through the coating thickness, but can lead to defects in the coating associated with macroparticle transfer and subsequent shadowing. Discussion of macroparticles from cathodic arc processing may be found in A. Anders, “Cathodic Arcs: From Fractal Spots to Energetic Condensation”, Springer (2008).
The cathodic arc deposited NiAl coatings may suffer from macroparticles that are believed to reduce oxidation performance. Additionally, the consumable source (i.e., cathode) used in the deposition process is difficult to manufacture due to the high melting temperature, large freezing range, and low ductility (i.e., brittleness) of the NiAl material.
U.S. Pat. No. 6,964,791 discloses a two-step method for supplying a coating onto a metallic substrate. The method includes disposing a first coating layer onto said substrate comprising nickel, zirconium, and aluminum, and thereafter, disposing onto the first coating layer, a second coating layer comprising at least 90 atomic percent aluminum. Subsequent processing leads to the formation of a substantially single-phase reacted coating layer comprising nickel aluminide with a B2 structure. The reacted coating layer includes a higher aluminum concentration at the outer surface and a lower aluminum concentration near the coating/substrate interface.
The teachings of U.S. Pat. No. 6,964,791 demonstrate that substantially pure aluminum deposited over NiAl(CrZr) coating layer exhibits high interdiffusion when heated to 1079° C. (1975° F.), thus covering the macroparticles mentioned above. However, the coating exhibits a higher aluminum surface level and an aluminum compositional gradient throughout the thickness of the coating.
Further improvements in coatings and coating processes are still sought, in particular, to reduce costs, ease manufacturing difficulties, and shorten coating times. In addition, improvements in alloying capabilities are sought due to the brittle nature of NiAl materials.